Oxygenates, such as ethers, have been a part of the U.S. gasoline strategy since the late 1970's. These materials reduce carbon monoxide emissions and unburned hydrocarbons in the exhaust of internal combustion engines. Another advantage of oxygenates is that they have relatively good blending characteristics. Some oxygenates have better blending characteristics than others. For example, the blending vapor pressures of methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), and tertiary amyl ether (TAME) are lower than methanol and ethanol, making them more attractive gasoline components.
MTBE or TAME can be produced by the addition of methanol to the corresponding isoolefin under etherification conditions. The reaction takes place in the presence of a catalyst at mild operating temperatures and pressures. The catalyst is usually a macrorecticular ion exchange resin based on a sulfonate styrene divinylbenzene copolymer.
It is not unusual for side reactions to occur during etherification reaction. For example, typical side reactions that can occur in an MTBE reactor include: (1) the formation of tertiary butyl alcohol (TBA) by isobutylene hydration; (2) the formation of di-isobutylene (DIB) by isobutylene dimerization; and the formation of dimethyl ether (DME) and water by methanol dehydration.
Downstream processing of the methyl tertiary butyl ether effluent usually includes separation of the ether products from the unconverted reactants, e.g., methanol. Effluent from the etherification reactor is usually passed to a fractionation tower where the methyl tertiary butyl ether product is removed from the bottom while side reaction products and unreacted reactants are separated as a raffinate overhead stream. The methanol contained in the raffinate is extracted with water by countercurrent, liquid-liquid extraction in a raffinate water wash tower. A methanol-containing stream leaves the bottom of the raffinate water wash tower and enters a methanol-water fractionation tower. A methanol-free raffinate stream leaves the top of the water wash tower and is directed to further downstream processing, e.g., alkylation. A water-containing stream free of methanol exits the bottom of the fractionation tower and is recycled for reuse in the raffinate water wash tower. A methanol-rich stream leaves the top of the fractionation tower and is recycled to the etherification reactor.
The most common source of isoolefinic hydrocarbons for use as a feedstock in an etherification process is the effluent from a fluid catalytic cracking (FCC) unit. FCC is a process for the conversion of straight-run atmospheric gas oil, vacuum gas oil, certain atmospheric residues, and heavy stocks recovered from other operations into high octane gasoline, light fuel oils and olefin-rich light gases. In simplified terms, the cracking reactions are carried out in a vertical reactor vessel in which vaporized oil rises and carries along with it, in intimate contact, small fluidized catalyst particles. The reactions are very rapid, and only a few seconds of contact time are necessary for most applications. In a petroleum refinery, the FCC unit typically processes 30-50% of the crude oil charged to the refinery.
Early FCC units were designed to operate on vacuum gas oils directly fractionated from crude oils. Typically, these vacuum oils came from high quality crude oils. Today, much of the high quality feedstock for FCC units have been depleted and modern FCC units process less favorable materials. These less favorable materials include a substantial amount of sulfur compounds, metal cations, and nitrogen compounds. As a result, the contaminant levels in the FCC effluent have been growing, particularly in the C.sub.3 -C.sub.5 effluent fraction. Without appropriate treatment, the contaminants in the C.sub.3 -C.sub.5 FCC effluent fraction can be transmitted to sensitive downstream processes where they reduce the effectiveness of downstream catalysts and create unfavorable by-product reactions in processes such as etherification.
The use of FCC effluent as a feedstock for an etherification process can pose problems due to the above-described impurities. The FCC effluent stream usually contains a significant amount of metal cations that can deactivate the etherification ion exchange resin catalyst by plug flow neutralization. Plug flow neutralization occurs when a strong cation such as sodium reacts with sulfonic acid groups on the catalyst. This type of neutralization begins at the reactor inlet bed and slowly moves along the length of the reactor over a period of time. The FCC effluent will also contain some nitrogen compounds, such as ammonia, light amines, dimethylformamide, and N-methyl-pyrrolidine.